The present invention generally relates to an apparatus and a method for wet cleaning wafers without ammonia vapor damage and more particularly, relates to an apparatus and a method for cleaning wafers with an ammonia-containing solution by trapping ammonia vapor in a cup-shaped container immersed in the cleaning solution such that ammonia vapor does not escape into the cleaning tank to attack wafers that are being cleaned in the tank.
Deionized water is frequently used in a wet bench process after a chemical process has been conducted on a semiconductor wafer. When residual chemical must be removed, deionized water rinse is used in a wet bench process for semiconductor wafer processing to perform two major functions of a quick dump rinse (QDR) and a cascade overflow rinse. Conventionally, the two functions are carried out in separate tanks in order to produce the desirable result. One of the major processing issues presented by the conventional dual-tank process is the particle re-deposition problem during a withdrawal step when cassettes are transported from a quick dump rinse tank to a cascade overflow tank. A second major issue is the large floor space required for accommodating the two tanks.
A conventional wet bench wafer cleaning process is shown in FIG. 1. The wet bench wafer cleaning process 10 for cleaning wafer 12 is carried out in six separate cleaning and rinsing tanks sequentially of a HF cleaning tank 14, a first quick dump rinse (QDR) tank 16, a SC-1 cleaning tank 18, a second quick dump rinse tank 20, a SC-2 cleaning tank 22 and a third quick dump rinse tank 24. The first HF cleaning tank is used to hold a diluted HF solution, for instance, at a concentration of 0.5% HF in H2O for removing a thin native oxide layer from the wafer surface. After the diluted HF cleaning process, the wafer 12 is rinsed in a first quick dump rinse tank 16 with deionized water. Wafer 12 is then cleaned in a second cleaning tank filled with SC-1 cleaning solution, i.e. a mixture of NH4OH, H2O2 and DI water at a ratio of 1:1:5. The SC-1 cleaning solution is used at a temperature between 70xcx9c80xc2x0 C. for a suitable time period. The wafer 12 is then rinsed again in a second quick dump rinse tank 20 that is filled with DI water. In the final stage of cleaning, the wafer 12 is cleaned in tank 22 filled with a cleaning solution of SC-2 which is a mixture of HCl, H2O2 and DI water at a ratio of 1:1:6. The wafer 12 is then rinsed in a third quick dump rinse tank 24 with DI water.
The wet bench wafer cleaning process 10 shown in FIG. 1 is conventionally used for pre-diffusion clean, pre-gate oxidation clean, pre-CVD clean, etc. For instance, in the ULSI fabrication of integrated devices, the conventional wet bench wafer cleaning process 10 can be advantageously used for wafer surface cleaning before a coating process in a CVD chamber or an oxidation process in a furnace.
Despite the fact that the conventional wet bench wafer cleaning process 10 is widely used, numerous processing difficulties of the process have been observed. For instance, during the SC-1 cleaning process carried out in tank 18, since SC-1 contains about 28% NH4OH which forms ammonia vapor in the tank cavity over the surface of the solution. When wafer 12 (or a whole wafer boat) is removed from the SC-1 cleaning solution and taken out of tank 18, ammonia vapor attacks the cleaned, bare wafer surface, i.e. the fresh silicon surface of the wafer. As a consequence, a defect known as xe2x80x9csilicon holexe2x80x9d occurs wherein craters in the silicon surface are formed due to the ammonia vapor attack.
Another processing difficulty encountered in the conventional wet bench cleaning process 10 is that, in order to perform the wafer cleaning process in a reliable and repeatable manner, the concentration and temperature of the cleaning solutions in the cleaning tanks must be maintained at constant values. For instance, during a typical wafer cleaning process, the SC-1 solution in tank 18 must be constantly replenished since the components in the SC-1 solution evaporates rapidly at a temperature higher than 50xc2x0 C. As a consequence, during a wafer cleaning process, fresh NH4OH must be constantly added into the SC-1 tank 18 in order to maintain a suitable concentration. A suitable method for monitoring the concentration of the SC-1 cleaning solution is by using an infrared analysis technique.
In a conventional method of adding NH4OH to the SC-1 tank 18, as shown in FIG. 2, wherein the tank 18 is further equipped with an outer tank (or an overflow tank) 26, the addition of the NH4OH liquid or any other ammonia-containing liquid further complicates the wafer cleaning process. The flow of NH4OH into the tank 18 causes agitation at the surface of the SC-1 liquid 28 and thus producing ammonia vapor 30 at or near the top surface of the SC-1 liquid 28. The ammonia vapor further presents a problem for attacking the bare, or freshly cleaned silicon wafer surface. This has been observed when NH4OH liquid was used to replenish either the SC-1 solution 28 in the SC-1 tank 18, or when the NH4OH liquid was used to replenish the SC-1 solution 32 in the outer tank 26.
It is therefore an object of the present invention to provide an apparatus for cleaning semiconductor wafers by wet bench that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for cleaning semiconductor wafers by wet bench without causing damages to the wafer surface by ammonia vapor.
It is a further object of the present invention to provide an apparatus for cleaning semiconductor wafers by wet bench without causing ammonia vapor damages to the wafer surface by minimizing the generation of ammonia vapor.
It is another further object of the present invention to provide an apparatus for cleaning semiconductor wafers by wet bench without causing ammonia vapor damages to the wafer surface by positioning a feed conduit for an ammonia-containing liquid through a bottom wall of a holding tank with outlet end of the conduit immersed in the cleaning solution.
It is another further object of the present invention to provide an apparatus for cleaning semiconductor wafers by wet bench without causing ammonia vapor damages to the wafer surface by positioning a feed conduit for an ammonia-containing liquid through a bottom wall of the holding tank with an outlet end of the conduit immersed in the cleaning solution and covered by a cup-shaped container such that any ammonia vapor generated is trapped in the container.
It is yet another object of the present invention to provide an apparatus for cleaning semiconductor wafers by wet bench without causing ammonia vapor damages to the wafer surface by positioning a feed conduit for an ammonia-containing liquid through a bottom wall of an outer tank with the liquid outlet end of the conduit immersed in a cleaning solution.
It is still another further object of the present invention to provide a method for cleaning semiconductor wafers by wet bench without causing ammonia vapor damages to the wafer surface by mounting a feed conduit for an ammonia-containing liquid through a bottom wall of a holding tank such that an outlet end of the feed conduit is immersed in the cleaning solution.
It is yet another further object of the present invention to provide a method for cleaning semiconductor wafers by wet bench without causing ammonia vapor damages to the wafer surface by mounting a cup-shaped container on top of a feed conduit for an ammonia-containing liquid such that any ammonia vapor generated is trapped by the cover that is immersed inside a cleaning solution.
In accordance with the present invention, an apparatus and a method for cleaning wafers by wet bench without causing ammonia vapor damages occurring to the wafer surface are provided.
In a preferred embodiment, a method for cleaning wafers by wet bench without causing ammonia vapor damages to the wafer surface can be carried out by the operating steps of first filling a tank with a quantity of a cleaning solution; positioning a conduit vertically through and with an outside wall sealingly engages a bottom wall of the tank with an outlet end of the conduit immersed in the cleaning solution; mounting a cup-shaped container in an upside-down position over the outlet end of the conduit in the cavity, the cup-shaped container retains ammonia vapor exiting the conduit and prevents the ammonia vapor from reaching an upper portion of the tank cavity; and flowing an ammonia-containing liquid through the conduit into the cavity of the tank to maintain a concentration of the cleaning solution.
The method for cleaning wafers by wet bench without ammonia vapor damage occurring to the wafer surface may further include the step of mounting an outer tank to the tank for receiving an overflow from the tank, or the step of filling the outer tank with an ammonia-containing liquid through a bottom wall of the outer tank. The method may further include the step of mounting the cup-shaped container to the conduit by mechanical means, or the step of mounting a particle trap to a second conduit for feeding the outer tank and for preventing contamination of the outer tank. The method may further include the step of flowing ammonium hydroxide through the conduit into the cavity of the tank to maintain a concentration of the cleaning solution, or the step of filling the tank with a cleaning solution mixture including ammonium hydroxide, H2O2 and deionized water. The method may further include the step of mounting the cup-shaped container to the conduit by plastic bolts that threadingly engage a sidewall of the cup-shaped container. The method may further include the step of providing the cup-shaped container in a dimension of between about 2 cm and about 10 cm in diameter and between about 2 cm and about 10 cm in height, or the step of immersing the cup-shaped container in the cleaning solution.
The present invention is further directed to a wet cleaning tank for cleaning semiconductor wafers by an ammonia-containing solution without causing ammonia vapor damage to the wafer surface including a tank body for holding a quantity of a cleaning solution therein; a conduit mounted through and vertical to a bottom wall of the tank body for feeding an ammonia-containing solution into the tank body through an outlet; and a cup-shaped container mounted in an upside-down position over the outlet of the conduit for blocking ammonia vapor from the ammonia-containing solution from reaching an upper cavity of the tank body.
The wet cleaning tank may further include an outer tank mounted to the tank body for receiving overflow from the tank body. The outer tank may be filled with an ammonia-containing liquid for flowing into the tank body for maintaining a concentration of the cleaning solution. The cup-shaped container may further include mechanical mounting means for mounting to the conduit, the mechanical mounting means may include plastic bolts threadingly engaging a sidewall of the cup-shaped container.
The wet cleaning tank may further include a second conduit for feeding the ammonia-containing liquid to the outer tank, the second conduit may be mounted through a bottom wall of the outer tank. The cleaning solution may include NH4OH, H2O2 and DIW (deionized water). The ammonia-containing liquid may include NH4OH. The cup-shaped container may have a dimension between about 2 cm and about 10 cm in diameter, and between about 2 cm and about 10 cm in height. The cup-shaped container may be mounted in an upside-down position with an air space therein for receiving and retaining the ammonia vapor.